In cellular-type radiotelephone communication systems using angle modulation, i.e., frequency modulation (FM) or phase modulation (PM), communication is-often impaired as a result of radio interference, such as, for example, co-channel and/or multipath interference. Co-channel interference is caused by spurious transmissions within the communication channel of concern. Multipath interference typically results when a signal is deflected off many structures, creating multiple reception paths, and is received as a number of multiple signals, each offset in time. Both types of interference can seriously inhibit the quality of communication.
Before this type of interference becomes unacceptable, a cellular communication system should, if possible, switch (hand-off) the communication channel to another cell to prevent disrupting the call. This requires, however, that the interference be accurately detected, and that the system knows the extent to which the co-channel and multipath interference are contributing factors. Consequently, the prior art reflects numerous methods for detecting radio interference and the extent to which it contributes to communication signal degradation.
One such system is described in U.S. Pat. No. 4,561,114, Kozono et al. and "Co-Channel Interference Measurement Method for Mobile-Communication," IEEE Transactions on Vehicular Technology, Vol VT-36, No 1, February 1987, both incorporated herein by reference. In these publications, it is proposed that co-channel interference can be detected in a narrow band communication system by looking for a high frequency beat signal (AM modulation) produced by an interfering co-channel signal. The interference is detected based on the frequency content of the envelope of the received signal to distinguish between shadow fading (discussed later in more detail) and co-channel interference. This is accomplished through a digital analysis technique which samples a first set of signals on a relatively infrequent basis, and samples an overlapping second set of signals on a relatively frequent basis. The extraction of a substantial number of samples from the first set of signals allows the system to average out interference due to fading, while the extraction of the second set of signals allows the system to obtain the average level of the high frequency beat signal. According to Kozono, the average level of the high frequency beat signal (AM modulation) indicates the amount of interference caused by co-channel interference.
Unfortunately, the system described above suffers from the disadvantage that it is liable to make false interference detections, i.e., false on fading. It will be appreciated by those skilled in the art that co-channel and multipath are not the only sources of high frequency spectrum components in the envelope of a received signal. On the contrary, in a mobile radiotelephone environment, characterized by the high speed of radiotelephone units, high frequency spectrum components due to fading in the Khz range must be anticipated. Yet another source of these high frequency spectrum components is due to the amplitude response of filters used in a receiver for intermediate frequency (IF) selectivity.
Since the Kozono detector detects an average level of the high frequency components in the envelope of a received signal, it is apt to respond to conditions other than interference. It would be extremely advantageous therefore to provide an interference detector and technique capable of overcoming the shortcomings of the prior art.